Android6.0的Looper源码分析（1）

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1 Looper简介

Android在Java标准线程模型的基础上，提供了消息驱动机制，用于多线程之间的通信。而其具体实现就是Looper。

Android Looper的实现主要包括了3个概念：Message，MessageQueue，Handler，Looper。其中Message就是表示一个可执行的任务。消息创建完毕通过消息处理器Handler在任意线程中发送添加至MessageQueue，最终在Looper线程逐个取出并调用handler.handleMessage()进行处理。

2 Looper的初始化

这里可以尝试分析Looper.java类的结构来推测Looper机制的实现原理。以下为Looper类的变量域：

//这里可以简单的将ThreadLocal类型的变量想象成一个Map,键值为线程号

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

// 注意下面的static表示sMainLooper归于Looper.Class

private static Looper sMainLooper; //注意static数据，进程间并非共享

//Looper的每个线程实例都有一个MessageQueue

final MessageQueue mQueue;

final Thread mThread;

第一个变量sThreadLocal为ThreadLocal<Looper>类型的变量，它主要由两个方法，set()和get()；这里通过泛型指定了需要线程赋值的变量类型为Looper。简单理解sThreadLocal .set()就是将当前线程的Looper副本值设定为指定值。sThreadLocal.get()将得到Looper实例在当前线程下的副本。（ThreadLocal的实现还有待研究，初步猜测其内部存在哈希Map，可以根据当前线程的线程号区分不同线程的变量）。通过ThreadLocal实现了线程级单例。

第二个变量为static的sMainLooper，存放的应该是主线程（即UI线程的Looper），类型设计为static，这样通过Looper.getMainLooper()的方法在任何线程都能获得该Looper，从而更新UI。

第三个参数为java层的Massage队列，Handler.sendMessage()就是将Message添加到此队列以供Looper.loop()。在接下来的分析将会发现，java层的MessageQueue的新建会导致Native层的NativeMessageQueue的创建，进而在导致Native层Looper的创建。

第四个参数，就是Looper所在线程的引用。

将一个线程改造成Looper线程很容易就可以实现，如下；

class LooperThread extends Thread {

public Handler mHandler;

public void run() {

Looper.prepare();

mHandler = new Handler() {//构造方法内部绑定了当前Looper线程

public void handleMessage(Message msg) {

// 在这里处理send进来的消息

}

};

Looper.loop();

}

首先分析Looper的准备工作prepare()。

public static void prepare() {

prepare(true);

}

private static void prepare(boolean quitAllowed) {//保证Looper的线程级单例

if (sThreadLocal.get() != null) {

throw new RuntimeException("Only one Looper may be created per thread");

}

sThreadLocal.set(new Looper(quitAllowed));//这里创建了Looper的线程单例

}

Looper线程单例的创建会导致MessageQueue的创建，MessageQueue内有一个Message类型的变量sMessages，因此可以想到MessageQueue在java层是通过链表实现的。以下为MessageQueue的构造函数：

MessageQueue(boolean quitAllowed) {

mQuitAllowed = quitAllowed;

//通过JNI调用了Native层的相关函数，导致了NativeMessageQueue的创建

mPtr = nativeInit();

}

可以看到MessageQueue在构造的时候通过JNI调用了Native层的C++函数，从而对Looper在Native层进行必要的初始化操作。同时java MessageQueue获得了一个指向Native层的指针mPtr，从而可以通过mPtr方便的调用底层的相关方法。NativeInit对应android_os_MessageQueue.cpp中的以下函数。

static jlong android_os_MessageQueue_nativeInit(JNIEnv* env, jclass clazz) {

//在Native层又创建了NativeMessageQueue

NativeMessageQueue* nativeMessageQueue = new NativeMessageQueue();

if (!nativeMessageQueue) {

jniThrowRuntimeException(env, "Unable to allocate native queue");

return 0;

}

nativeMessageQueue->incStrong(env);

//这里的返回给java层的mPtr，因此mPtr实际上是Java MessageQueue与

//nativeMessageQueue的桥梁，这里比老版本实现更为简洁

return reinterpret_cast<jlong>(nativeMessageQueue);

}

此时Java层和Native层的MessageQueue被mPtr连接起来了，NativeMessageQueue只是java层MessageQueue在Ntive层的体现，其本身并没有实现Queue的数据结构，而是从其父类MessageQueue中继承了mLooper变量。与java层类似，这个Looper也是线程级单例。以下为NativeMessageQueue的构造函数：

NativeMessageQueue::NativeMessageQueue() :

mPollEnv(NULL), mPollObj(NULL), mExceptionObj(NULL) {

mLooper = Looper::getForThread();

if (mLooper == NULL) {

mLooper = new Looper(false);//在Native层创建了Looper对象

Looper::setForThread(mLooper);//同样是线程级单例

}

可以看到在Java层Looper的创建导致了MessageQueue的创建，而在Native层则刚好相反：NativeMessageQueue的创建导致了Looper的创建。而且Native层的Looper创建和Java层的也完全不一样。它利用了Linux的epoll机制监测了Input的fd和唤醒fd。从功能上来讲，这个唤醒fd才是真正处理java Message和Native Message的钥匙。（注意5.0以上版本Looper的定义在System/core下）。

Looper::Looper(bool allowNonCallbacks) :

mAllowNonCallbacks(allowNonCallbacks), mSendingMessage(false),

mPolling(false), mEpollFd(-1), mEpollRebuildRequired(false),

mNextRequestSeq(0), mResponseIndex(0), mNextMessageUptime(LLONG_MAX) {

//这是linux后来才有的东西，负责线程通信,替换了老版本的pipe

mWakeEventFd = eventfd(0, EFD_NONBLOCK);

LOG_ALWAYS_FATAL_IF(mWakeEventFd < 0, "Could not make wake event fd. errno=%d", errno);

AutoMutex _l(mLock);

rebuildEpollLocked();

}

进入rebuildEpollLocked

void Looper::rebuildEpollLocked() {

// Close old epoll instance if we have one.

if (mEpollFd >= 0) {

#if DEBUG_CALLBACKS

ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);

#endif

close(mEpollFd);

}

// Allocate the new epoll instance and register the wake pipe.

//采用linux的Epoll,与Select功能其实有点类似

mEpollFd = epoll_create(EPOLL_SIZE_HINT);

LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno);

struct epoll_event eventItem;

memset(& eventItem, 0, sizeof(epoll_event)); // 清空

eventItem.events = EPOLLIN;//关注EPOLLIN事件，也就是可读

eventItem.data.fd = mWakeEventFd;//设置Fd

// 将mWakeEventFd的event添加到监听队列,这里其实只是为epoll_ctl放置一个唤醒机制

int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);

LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance. errno=%d",

errno);

//这里主要添加的是Input事件如键盘，传感器输入，这里基本上由系统负责，很少主动去添加

for (size_t i = 0; i < mRequests.size(); i++) {

const Request& request = mRequests.valueAt(i);

struct epoll_event eventItem;

request.initEventItem(&eventItem);

int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);

if (epollResult < 0) {

ALOGE("Error adding epoll events for fd %d while rebuilding epoll set, errno=%d",

request.fd, errno);

}

这里一定要明白的是，添加的这些fd除了mWakeEventFd负责解除阻塞让程序继续运行，从而处理Native Message和Java Message外，其他fd与Message的处理其实，毫无关系（知道这点非常重要）。此时Java层与Native层的联系如下图所示：

3 创建消息并发送消息

创建消息和发送消息一般是在Looper线程之外的另一个线程通过Handler发送。以下是Handler的满参构造方法。

public Handler(Callback callback, boolean async) {

if (FIND_POTENTIAL_LEAKS) {//调试接口，默认为false

final Class<? extends Handler> klass = getClass();

if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&

(klass.getModifiers() & Modifier.STATIC) == 0) {

Log.w(TAG, "The following Handler class should be static or leaks might occur: " +

klass.getCanonicalName());

}

//Handler绑定当前线程的Looper实例

mLooper = Looper.myLooper();

if (mLooper == null) {

throw new RuntimeException(

"Can't create handler inside thread that has not called Looper.prepare()");

}

mQueue = mLooper.mQueue;//sendMessage的目标队列就是Looper的MessageQueue

mCallback = callback;//Handler指定callback

mAsynchronous = async;//是否异步

}

在每一个Handler的构造过程中，Handler通过“mLooper =Looper.myLooper();”悄悄的持有了当前所在的looper线程的一个引用。我们已经知道每个Looper都会有一个MessageQueue，这样Handler，Looper，MessageQueue就被关联起来了。

利用Handler发送消息之前需要新建一个Message。获取Message一般可以采用Message类的static方法obtain()。此方法有很多重载方法，零参实现如下（多参重载只是对零参时未赋值的变量进行了赋值）

public static Message obtain() {

synchronized (sPoolSync) {

if (sPool != null) {

Message m = sPool;

sPool = m.next;

m.next = null;

m.flags = 0; // clear in-use flag

sPoolSize--;

return m;

}

return new Message();

}

接着就可以调用Handler（非Looper线程持有Handler引用）的sendMessage（msg）方法。前面已经提到，Handler内部持有一个Looper的引用，Looper内部有一个MessageQueue。这样就实现了线程间的消息传递。当然除了sendMessage（msg）之外还有其他类似的发送消息的函数。其本质就是往MessageQueue里面添加Message。这里就不详述了。

特别要指出的是Looper.loop（）在消息队列为空的情况下并不是阻塞在这个MessageQueue上，而是阻塞在Native层的epoll_wait上面。这样会存在很多问题，一个最为重要的问题就是如果在阻塞的时候，突然接收到java Message，程序怎么立马去处理这个Message？前面提到epoll监听了Input的fd和mWakeEventFd。答案就在mWakeEventFd。

先来看每个sendMessage()或其他Send方法都会最终调用以下的这个方法。

boolean enqueueMessage(Message msg, long when) {

if (msg.target == null) {

throw new IllegalArgumentException("Message must have a target.");

}

if (msg.isInUse()) {

throw new IllegalStateException(msg + " This message is already in use.");

}

synchronized (this) {

if (mQuitting) {

IllegalStateException e = new IllegalStateException(

msg.target + " sending message to a Handler on a dead thread");

Log.w(TAG, e.getMessage(), e);

msg.recycle();

return false;

}

msg.markInUse();

msg.when = when;

Message p = mMessages;

boolean needWake;

if (p == null || when == 0 || when < p.when) {

// New head, wake up the event queue if blocked.

msg.next = p;

mMessages = msg;

needWake = mBlocked;

} else {

// Inserted within the middle of the queue. Usually we don't have to wake

// up the event queue unless there is a barrier at the head of the queue

// and the message is the earliest asynchronous message in the queue.

needWake = mBlocked && p.target == null && msg.isAsynchronous();

Message prev;

for (;;) {

prev = p;

p = p.next;

if (p == null || when < p.when) {

break;

}

if (needWake && p.isAsynchronous()) {

needWake = false;

}

msg.next = p; // invariant: p == prev.next

prev.next = msg;

}

// We can assume mPtr != 0 because mQuitting is false.

if (needWake) {

nativeWake(mPtr);

}

return true;

}

可以看到以上函数才是真正添加Message的实干函数。在每次添加完毕之后都在需needWake的时候去调用NativeWake(mPtr)。我们已经知道mPtr指向了Native层的NativeMessageQueue。NativeWake(mPtr)最终调用了该类的wake()方法。此方法向mWakeEventFd写入了一个字节的内容。到底是什么内容并不重要，重要的是fd存在内容了，换句话说就是mWakeEventFd可读了！因此epoll_wait返回。首先遍历Native消息队列（此时基本上为空遍历），接着遍历活动fd，这里只有一个活动fd就是mWakeEventFd，读掉这一个字节的数据解除掉mWakeEventFd的可读状态。此时mWakeEventFd功成身退。程序已经从阻塞状态解除了出来。程序返回到java层的MessageQueue.next()函数中，next函数返回即从MessageQueue中返回此msg，以做后续的处理。

首先来看Looper.loop()。

public static void loop() {

final Looper me = myLooper();

if (me == null) {

throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");

}

final MessageQueue queue = me.mQueue;

// Make sure the identity of this thread is that of the local process,

// and keep track of what that identity token actually is.

Binder.clearCallingIdentity();

final long ident = Binder.clearCallingIdentity();

for (;;) {//无限循环直到quit()

Message msg = queue.next();//获取下一个java Message

if (msg == null) {

// No message indicates that the message queue is quitting.

return;

}

// This must be in a local variable, in case a UI event sets the logger

Printer logging = me.mLogging;

if (logging != null) {

logging.println(">>>>> Dispatching to " + msg.target + " " +

msg.callback + ": " + msg.what);

}

msg.target.dispatchMessage(msg);//java层的Message处理在这里

if (logging != null) {

logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);

}

// Make sure that during the course of dispatching the

// identity of the thread wasn't corrupted.

final long newIdent = Binder.clearCallingIdentity();

if (ident != newIdent) {

Log.wtf(TAG, "Thread identity changed from 0x"

+ Long.toHexString(ident) + " to 0x"

+ Long.toHexString(newIdent) + " while dispatching to "

+ msg.target.getClass().getName() + " "

+ msg.callback + " what=" + msg.what);

}

msg.recycleUnchecked();

}

这里直接进入MessageQueue.next()

Message next() {

// Return here if the message loop has already quit and been disposed.

// This can happen if the application tries to restart a looper after quit

// which is not supported.

final long ptr = mPtr;

if (ptr == 0) {

return null;

}

int pendingIdleHandlerCount = -1; // -1 only during first iteration

int nextPollTimeoutMillis = 0;//这个参数向Native层epoll_wait指定时超时时间

for (;;) {

if (nextPollTimeoutMillis != 0) {//此处作用有待研究

Binder.flushPendingCommands();

}

nativePollOnce(ptr, nextPollTimeoutMillis);//一般都是阻塞在这个函数

synchronized (this) {

// Try to retrieve the next message. Return if found.

final long now = SystemClock.uptimeMillis();

Message prevMsg = null;

Message msg = mMessages;

if (msg != null && msg.target == null) {

// Stalled by a barrier. Find the next asynchronous message in the queue.

do {

prevMsg = msg;

msg = msg.next;

} while (msg != null && !msg.isAsynchronous());

}

if (msg != null) {

if (now < msg.when) {

// Next message is not ready. Set a timeout to wake up when it is ready.

nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);

} else {

// Got a message.

mBlocked = false;

if (prevMsg != null) {

prevMsg.next = msg.next;

} else {

mMessages = msg.next;

}

msg.next = null;

if (DEBUG) Log.v(TAG, "Returning message: " + msg);

msg.markInUse();

return msg;

}

} else {

// No more messages.

nextPollTimeoutMillis = -1;

}

// Process the quit message now that all pending messages have been handled.

if (mQuitting) {

dispose();

return null;

}

// If first time idle, then get the number of idlers to run.

// Idle handles only run if the queue is empty or if the first message

// in the queue (possibly a barrier) is due to be handled in the future.

if (pendingIdleHandlerCount < 0

&& (mMessages == null || now < mMessages.when)) {

pendingIdleHandlerCount = mIdleHandlers.size();

}

if (pendingIdleHandlerCount <= 0) {

// No idle handlers to run. Loop and wait some more.

mBlocked = true;

continue;

}

if (mPendingIdleHandlers == null) {

mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];

}

mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);

}

// Run the idle handlers.

// We only ever reach this code block during the first iteration.

for (int i = 0; i < pendingIdleHandlerCount; i++) {

final IdleHandler idler = mPendingIdleHandlers[i];

mPendingIdleHandlers[i] = null; // release the reference to the handler

boolean keep = false;

try {

keep = idler.queueIdle();

} catch (Throwable t) {

Log.wtf(TAG, "IdleHandler threw exception", t);

}

if (!keep) {

synchronized (this) {

mIdleHandlers.remove(idler);

}

// Reset the idle handler count to 0 so we do not run them again.

pendingIdleHandlerCount = 0;

// While calling an idle handler, a new message could have been delivered

// so go back and look again for a pending message without waiting.

nextPollTimeoutMillis = 0;

}

上面函数中最为重要的变量为nextPollTimeoutMillis。这个参数为Native层的epoll_wait指定了超时时间。为什么会存在这个epoll_wait超时时间呢？不是已经有一个mWakeEventFd已经可以唤醒epoll_wait了么？回答这个问题需要对Message加以分析，存在多种Message，其中一种Message为需要立即执行的消息。这样的消息通过mWakeEventFd唤醒就可以了。另一种消息是延时消息，或者是在指定时间执行的消息。这样的消息添加到MessageQueue后一般不需要立即执行，而是等一段时间才会去执行，通过一些必要的计算给epoll_wait()指定超时时间可以使得在需要执行这些定时任务的时候epoll_wait()返回。此函数就是实现了这样的逻辑。

接着上面的之前的分析，Looper.loop()调用MessageQueue.next()。next()调用NativePollOnce从而进入Native层处理input和Native Message。NativePollOnce经过几次转调最终会落在mLooper.PollOnce()，如下：

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {

int result = 0;

for (;;) {//首先对fd对应的的responses进行处理，后面会发现responses里都是活动fd

while (mResponseIndex < mResponses.size()) {

const Response& response = mResponses.itemAt(mResponseIndex++);

int ident = response.request.ident;

if (ident >= 0) {//这里大于0标示没有指定callback直接返回即可，有为-2

int fd = response.request.fd;

int events = response.events;

void* data = response.request.data;

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - returning signalled identifier %d: "

"fd=%d, events=0x%x, data=%p",

this, ident, fd, events, data);

#endif

if (outFd != NULL) *outFd = fd;

if (outEvents != NULL) *outEvents = events;

if (outData != NULL) *outData = data;

return ident;

}

if (result != 0) {//注意这里处于循环内部，改变result的值是在后面的pollInner

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - returning result %d", this, result);

#endif

if (outFd != NULL) *outFd = 0;

if (outEvents != NULL) *outEvents = 0;

if (outData != NULL) *outData = NULL;

return result;

}

result = pollInner(timeoutMillis);//内部epoll_wait

}

接着进入pollInner

int Looper::pollInner(int timeoutMillis) {

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - waiting: timeoutMillis=%d", this, timeoutMillis);

#endif

// Adjust the timeout based on when the next message is due.

if (timeoutMillis != 0 && mNextMessageUptime != LLONG_MAX) {

nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);

int messageTimeoutMillis = toMillisecondTimeoutDelay(now, mNextMessageUptime);

if (messageTimeoutMillis >= 0

&& (timeoutMillis < 0 || messageTimeoutMillis < timeoutMillis)) {

timeoutMillis = messageTimeoutMillis;

}

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - next message in %" PRId64 "ns, adjusted timeout: timeoutMillis=%d",

this, mNextMessageUptime - now, timeoutMillis);

#endif

}

// Poll.

int result = POLL_WAKE;

mResponses.clear();

mResponseIndex = 0;

// We are about to idle.

mPolling = true;

struct epoll_event eventItems[EPOLL_MAX_EVENTS];

int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

// No longer idling.

mPolling = false;

// 获得锁，在Native Message的处理和添加逻辑上需要同步

mLock.lock();

//如果需要，重建epoll

if (mEpollRebuildRequired) {

mEpollRebuildRequired = false;

rebuildEpollLocked();

goto Done;

}

// Check for poll error.

if (eventCount < 0) {

if (errno == EINTR) {

goto Done;

}

ALOGW("Poll failed with an unexpected error, errno=%d", errno);

result = POLL_ERROR;

goto Done;

}

// epoll超时

if (eventCount == 0) {

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - timeout", this);

#endif

result = POLL_TIMEOUT;//此值返回PollOnce，从而导致java定时Message执行

goto Done;

}

// Handle all events.

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - handling events from %d fds", this, eventCount);

#endif

//首先处理活动的input设备和mWakeEventFd

for (int i = 0; i < eventCount; i++) {

int fd = eventItems[i].data.fd;

uint32_t epollEvents = eventItems[i].events;

if (fd == mWakeEventFd) {//若果是唤醒fd有反应

if (epollEvents & EPOLLIN) {

awoken();//内部就是read,从而使fd可读状态被清除

} else {

ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);

}

} else {//其他input fd处理，其实就是讲活动fd放入到responses队列中，等待处理

ssize_t requestIndex = mRequests.indexOfKey(fd);

if (requestIndex >= 0) {

int events = 0;

if (epollEvents & EPOLLIN) events |= EVENT_INPUT;

if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;

if (epollEvents & EPOLLERR) events |= EVENT_ERROR;

if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP;

pushResponse(events, mRequests.valueAt(requestIndex));

} else {

ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "

"no longer registered.", epollEvents, fd);

}

Done: ;

// 这里应该是处理Native层的Message

mNextMessageUptime = LLONG_MAX;

while (mMessageEnvelopes.size() != 0) {

nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);

const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);

if (messageEnvelope.uptime <= now) {

// Remove the envelope from the list.

// We keep a strong reference to the handler until the call to handleMessage

// finishes. Then we drop it so that the handler can be deleted *before*

// we reacquire our lock.

{ // obtain handler

sp<MessageHandler> handler = messageEnvelope.handler;

Message message = messageEnvelope.message;

mMessageEnvelopes.removeAt(0);

mSendingMessage = true;

mLock.unlock();

#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS

ALOGD("%p ~ pollOnce - sending message: handler=%p, what=%d",

this, handler.get(), message.what);

#endif

handler->handleMessage(message);//处理Native Message

} // release handler

mLock.lock();

mSendingMessage = false;

result = POLL_CALLBACK;

} else {

// The last message left at the head of the queue determines the next wakeup time.

mNextMessageUptime = messageEnvelope.uptime;

break;

}

// Release lock.

mLock.unlock();

// 处理之前添加进responses的活动Input设备

for (size_t i = 0; i < mResponses.size(); i++) {

Response& response = mResponses.editItemAt(i);

if (response.request.ident == POLL_CALLBACK) {

int fd = response.request.fd;

int events = response.events;

void* data = response.request.data;

#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS

ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",

this, response.request.callback.get(), fd, events, data);

#endif

// Invoke the callback. Note that the file descriptor may be closed by

// the callback (and potentially even reused) before the function returns so

// we need to be a little careful when removing the file descriptor afterwards.

//这里处理了有callback的fd,没有fd的处理可以推后到下次循环的pollOnce

int callbackResult = response.request.callback->handleEvent(fd, events, data);

if (callbackResult == 0) {

removeFd(fd, response.request.seq);

}

// Clear the callback reference in the response structure promptly because we

// will not clear the response vector itself until the next poll.

response.request.callback.clear();

result = POLL_CALLBACK;

}

return result;

}

下面是Looper的处理结构图。关键在于epoll。

这里很明显涉及到3类消息的处理：

1，Java层的Message

2，Native层的Message

3，活动fd指向的Input设备

下面将对着三类消息一一进行分析。

4 Java层 Message的处理

首先需要明确的是Java层Message的执行时机。在上一节的分析中已经分析过了，它是在Native层Message和fd之后。Looper.loop()阻塞的位置在MassageQueue.next()->pollOnce()->pollInner()->epoll_wait()。

1，如果三类消息都为空，此时Java层send进来一个msg。sendMessage()将调用NativeWake唤醒epoll_wait()。从而回到Java层处理该msg。

2，如果只有Java层有msg，且为定时任务，sendMessage时唤醒epoll_wait()。在下一次循环中为epoll_wait设置超时时间。（实际上逻辑更为复杂）。

3，在循环时添加Java Message。epoll_wait立即返回。Msg在下一次循环被处理。

Java层Message的发送和处理流程大致如下图所示：

5 Native层 Message的处理

Native层Message的发送和处理流程大致如下图所示：

从图中可以发现，Native消息的发送过程和处理与java层Message的处理比较类似。都是在任意线程中新建一个Message，然后sendMessage()，所不同的是Native层的Looper没有Handler，因此sendMessage只能通过Looper.sendMessage()。并且需要在SendMessage（）时为该Message指定处理该Message的MessageHandler。而且Native层MessageQueue的实现mMessageEnvelopes本质上是Vector，这一点和Java层MessageQueue是不同的。同样需要在sendMessage()的时候wake()。逻辑和Java层类似就不赘述了。

6 活动fd对应的Input设备的处理

这类消息由epoll直接监听fd，当input设备有活动时，epoll_wait()检测到对应的fd可读（或可写）。从而对fd做处理。这类消息的处理比较分散，首先来看pollInner()。

int Looper::pollInner(int timeoutMillis) {

……

int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_EVENTS, timeoutMillis);

……

for (int i = 0; i < eventCount; i++) {

int fd = eventItems[i].data.fd;

uint32_t epollEvents = eventItems[i].events;

if (fd == mWakeEventFd) {

if (epollEvents & EPOLLIN) {

awoken();

} else {

ALOGW("Ignoring unexpected epoll events 0x%x on wake event fd.", epollEvents);

}

} else {

ssize_t requestIndex = mRequests.indexOfKey(fd);

if (requestIndex >= 0) {

int events = 0;

if (epollEvents & EPOLLIN) events |= EVENT_INPUT;

if (epollEvents & EPOLLOUT) events |= EVENT_OUTPUT;

if (epollEvents & EPOLLERR) events |= EVENT_ERROR;

if (epollEvents & EPOLLHUP) events |= EVENT_HANGUP;

//将活动的fd对应mRequests包装成responses队列

pushResponse(events, mRequests.valueAt(requestIndex));

} else {

ALOGW("Ignoring unexpected epoll events 0x%x on fd %d that is "

"no longer registered.", epollEvents, fd);

}

……

}

// 带callback的responses处理

for (size_t i = 0; i < mResponses.size(); i++) {

Response& response = mResponses.editItemAt(i);

if (response.request.ident == POLL_CALLBACK) {

int fd = response.request.fd;

int events = response.events;

void* data = response.request.data;

#if DEBUG_POLL_AND_WAKE || DEBUG_CALLBACKS

ALOGD("%p ~ pollOnce - invoking fd event callback %p: fd=%d, events=0x%x, data=%p",

this, response.request.callback.get(), fd, events, data);

#endif

// Invoke the callback. Note that the file descriptor may be closed by

// the callback (and potentially even reused) before the function returns so

// we need to be a little careful when removing the file descriptor afterwards.

int callbackResult = response.request.callback->handleEvent(fd, events, data);

if (callbackResult == 0) {

removeFd(fd, response.request.seq);

}

// Clear the callback reference in the response structure promptly because we

// will not clear the response vector itself until the next poll.

response.request.callback.clear();

result = POLL_CALLBACK;

}

return result;

}

可以看到，对于活跃fd已经包含了callback的response，直接调用了此callback的HandlerEvent()函数。那对于没有指定Callback的活动responses在那处理呢？在下一次训话中的PollOnce（）。也就是下一次epoll_wait()之前。

int Looper::pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData) {

int result = 0;

for (;;) {

while (mResponseIndex < mResponses.size()) {

const Response& response = mResponses.itemAt(mResponseIndex++);

int ident = response.request.ident;

if (ident >= 0) {//这里大于0标示没有指定callback直接返回即可，有为-2

int fd = response.request.fd;

int events = response.events;

void* data = response.request.data;

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - returning signalled identifier %d: "

"fd=%d, events=0x%x, data=%p",

this, ident, fd, events, data);

#endif

if (outFd != NULL) *outFd = fd;

if (outEvents != NULL) *outEvents = events;

if (outData != NULL) *outData = data;

return ident;//对没有callback的response直接返回ident(“没有callback”)

}

if (result != 0) {

#if DEBUG_POLL_AND_WAKE

ALOGD("%p ~ pollOnce - returning result %d", this, result);

#endif

if (outFd != NULL) *outFd = 0;

if (outEvents != NULL) *outEvents = 0;

if (outData != NULL) *outData = NULL;

return result;

}

result = pollInner(timeoutMillis);

}

注意pollOnce传入此函数的后三个参数为指针，因此也可以被认为是“返回值”，上层由此获得了一个活动fd的副本，以做后续处理。而此活动fd被responses.clear()掉。

接着还是来继续分析自带callback的request。这里面临两个问题：1，谁添加了这些request？2，这些request的callback->handleEvent（）到底指向了那个函数？

对于第一个为题，可从后往前分析。epoll使用的是fd。这些fd在NativeInit中具体一点就是在Native Looper的构建中被添加进epoll监听队列中，如下

void Looper::rebuildEpollLocked() {

// Close old epoll instance if we have one.

if (mEpollFd >= 0) {

#if DEBUG_CALLBACKS

ALOGD("%p ~ rebuildEpollLocked - rebuilding epoll set", this);

#endif

close(mEpollFd);

}

// Allocate the new epoll instance and register the wake pipe.

mEpollFd = epoll_create(EPOLL_SIZE_HINT);

LOG_ALWAYS_FATAL_IF(mEpollFd < 0, "Could not create epoll instance. errno=%d", errno);

struct epoll_event eventItem;

memset(& eventItem, 0, sizeof(epoll_event)); // zero out unused members of data field union

eventItem.events = EPOLLIN;

eventItem.data.fd = mWakeEventFd;

int result = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, mWakeEventFd, & eventItem);

LOG_ALWAYS_FATAL_IF(result != 0, "Could not add wake event fd to epoll instance. errno=%d",

errno);

//就是这里

for (size_t i = 0; i < mRequests.size(); i++) {

const Request& request = mRequests.valueAt(i);

struct epoll_event eventItem;

request.initEventItem(&eventItem);

int epollResult = epoll_ctl(mEpollFd, EPOLL_CTL_ADD, request.fd, & eventItem);

if (epollResult < 0) {

ALOGE("Error adding epoll events for fd %d while rebuilding epoll set, errno=%d",

request.fd, errno);

}

从以上程序可以发现这些fd都是mRequests中取出来的。而mRequests由Looper.addFd()添加。查看此函数的调用者发现，很多地方都有调用此函数。因此推测在Native层可以直接使用此函数，向epoll添加监听fd。那java层能向epoll添加fd么？发现NativeInit在Native层对应的函数android_os_MessageQueue_nativeInit有一个邻居如下。

static void android_os_MessageQueue_nativeSetFileDescriptorEvents(JNIEnv* env, jclass clazz,

jlong ptr, jint fd, jint events) {

NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);

nativeMessageQueue->setFileDescriptorEvents(fd, events);

}

进入setFileDescriptorEvents()

void NativeMessageQueue::setFileDescriptorEvents(int fd, int events) {

if (events) {//从这里判断是添加还是删除

int looperEvents = 0;

if (events & CALLBACK_EVENT_INPUT) {

looperEvents |= Looper::EVENT_INPUT;

}

if (events & CALLBACK_EVENT_OUTPUT) {

looperEvents |= Looper::EVENT_OUTPUT;

}

mLooper->addFd(fd, Looper::POLL_CALLBACK, looperEvents, this,

reinterpret_cast<void*>(events));//添加fd，this指明callback为类自己

} else {

mLooper->removeFd(fd);//这里删除fd

}

因此在Java层也是可以向epoll添加fd的

private void updateOnFileDescriptorEventListenerLocked(FileDescriptor fd, int events,

OnFileDescriptorEventListener listener) {

final int fdNum = fd.getInt$();

int index = -1;

FileDescriptorRecord record = null;

if (mFileDescriptorRecords != null) {

index = mFileDescriptorRecords.indexOfKey(fdNum);

if (index >= 0) {

record = mFileDescriptorRecords.valueAt(index);

if (record != null && record.mEvents == events) {

return;

}

if (events != 0) {

events |= OnFileDescriptorEventListener.EVENT_ERROR;

if (record == null) {

if (mFileDescriptorRecords == null) {

mFileDescriptorRecords = new SparseArray<FileDescriptorRecord>();

}

record = new FileDescriptorRecord(fd, events, listener);

mFileDescriptorRecords.put(fdNum, record);

} else {

record.mListener = listener;

record.mEvents = events;

record.mSeq += 1;

}

nativeSetFileDescriptorEvents(mPtr, fdNum, events);//添加或删除fd

} else if (record != null) {

record.mEvents = 0;

mFileDescriptorRecords.removeAt(index);//猜测是java层的fd记录

}

由于在addFd时候指定自己也就是this是callback，因此到最后该fd处理的时候会进入NativeMessageQueue的handlerEvent()方法。

int NativeMessageQueue::handleEvent(int fd, int looperEvents, void* data) {

int events = 0;

if (looperEvents & Looper::EVENT_INPUT) {

events |= CALLBACK_EVENT_INPUT;

}

if (looperEvents & Looper::EVENT_OUTPUT) {

events |= CALLBACK_EVENT_OUTPUT;

}

if (looperEvents & (Looper::EVENT_ERROR | Looper::EVENT_HANGUP | Looper::EVENT_INVALID)) {

events |= CALLBACK_EVENT_ERROR;

}

int oldWatchedEvents = reinterpret_cast<intptr_t>(data);

int newWatchedEvents = mPollEnv->CallIntMethod(mPollObj,//调用java层代码

gMessageQueueClassInfo.dispatchEvents, fd, events);

if (!newWatchedEvents) {

return 0; // unregister the fd

}

if (newWatchedEvents != oldWatchedEvents) {

setFileDescriptorEvents(fd, newWatchedEvents);

}

return 1;

}

需要注意的是gMessageQueueClassInfo指向了java层的MessageQueue，因此MessageQueue的dispatchEvents()方法被调用。Message在java层被处理。

当然在Native层就可以实现fd的添加和处理。貌似这也是主要的途径。Android6.0有好些个专门的类处理Input设备。如android_view_InputQueue、android_view_InputEventSender、android_view_InputEventReceiver等等。这里就不详述了。留待以后研究。

原文地址： http://blog.csdn.net/a34140974/article/details/50638089

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